Penelope left home at 11.30 a.m. and drove towards the town. 3 hours later, her grandfather left their home and drove towards the same town at 90 km/h. When the grandfather reached the town at 6.30 p.m., Penelope had another 3 km to travel. What was Penelope’s average speed? Express your answer in mixed number.
Time that the Penelope's grandfather started driving = 2.30 p.m.
Duration that Penelope's grandfather drove from 2.30 p.m. to 6.30 p.m. = 4 h

Distance that Penelope's grandfather travelled
= 4 x 90
= 360 km

Distance that Penelope travelled by the time her grandfather reached the city
= 360 - 3
= 357 km

Time that Penelope took to travel 357 km = 7 h

Penelope's average speed
= 357 ÷ 7
= 51 km/h

Answer(s): 51 km/h